1. Field of the Invention
This invention relates to radar systems utilizing Moving-Target-Indication (MTI) for signal detection. More particularly, this invention relates to an arrangement useful for attenuating clutter echoes of fixed targets (non-doppler shifted) to reveal the echoes of a moving target (doppler shifted).
2. Description of Related Art
The receiver of an MTI radar system receives large non doppler-shifted return echoes from stationary objects in addition to receiving doppler-shifted return echoes from targets of interest. These non doppler-shifted return echoes are referred to as "clutter". An A/D converter typically digitizes the output of the receiver and a system processor uses digital filtering techniques to separate the doppler-shifted target echo from the non doppler-shifted clutter.
The magnitude of the clutter signal, however, often exceeds the magnitude of the target return signal by as much as 90 dB. The ensuing problem is that it is difficult to achieve the receiver linearity (in the form of intermodulation suppression) and the A/D converter dynamic range (in terms of number of bits in the output) necessary to recognize doppler shifted echoes amid the clutter. If it were possible to reject the non-doppler shifted clutter before the receiver amplifies the signal and before the A/D converter digitizes the receiver output, the stringent requirements on receiver linearity and A/D dynamic range could be relaxed. Each additional 6 dB of clutter rejected before A/D conversion would reduce the required number of A/D output bits by one.
FIG. 1 depicts a single pole clutter canceller described by Skolnik in "Introduction to Radar Systems". This clutter canceller utilizes delay line 103 to remove clutter at IF and before A/D conversion. A radar IF input signal 100 is split by power divider 102 into two signals. One of the signals is put through delay line 103 before being supplied to combiner 104. The other signal is put directly to combiner 104. Combiner 104 subtracts one of the signals from the other to output the resulting clutter cancelled IF output signal 101.
Note that delay line 103 has a propagation delay precisely equal to the amount of time between successive radar pulse transmissions. This period of time is referred to as the interpulse period. Delay line 103 delays the return echo from a first interpulse period for precisely one interpulse period of time. This delayed echo is then subtracted from the next return echo received by the radar receiver in the next interpulse period.
Because the clutter component of the return echo contains no doppler shift, it has the same phase from one interpulse period to the next. Subtraction of one interpulse period from the next, therefore, results in the cancellation of the clutter component. The target return component of the return echo, however, contains a doppler shift and a phase difference exists between target return components of different interpulse periods. Subtraction of one interpulse period from the next, therefore, does not result in cancellation of the target return component. This occurs because the IF center frequency at which subtraction takes place is an integer multiple of the system pulse repetition interval (reciprocal of interpulse period). If the propagation delay of delay line 103 does not precisely correspond to the interpulse period, imperfect time alignment occurs and complete cancellation of the clutter in combiner 104 is prevented.
Previous IF clutter cancellers have used surface acoustic wave (SAW) delay lines and bulk acoustic wave (BAW) delay lines. The signal going through the delay line, however, always propagates through a different signal path than does the non-delayed signal. One reason that these clutter cancellers have not seen widespread use is that the effects of temperature, ageing, manufacturing tolerances, and other factors affecting phase and delay are different on the two paths. Clutter cancellers utilizing different signal paths, therefore, achieve only limited clutter cancellation and limited long term stability. Another problem with these clutter cancellers involves the fact that the delays of their delay lines are fixed and not programmable. The use of these clutter cancellers in radar systems which use several different interpulse periods requires that a separate delay line be provided for each different interpulse period used. This type of clutter canceller has therefore not found widespread use.
A new device called an acoustic charge transport (ACT) delay line has recently been developed. The ACT device utilizes a combination of surface acoustic wave (SAW) technology and field effect transistor (FET) technology to affect a monolithic GaAs RF delay line. The ACT is basically a four terminal device in its most fundamental form. The ACT detailed in FIG. 3 has a sampler drive signal input D1, an input signal input A1T1, an interrupt field input INT1, and one or more output taps A1T2.
When a high power, constant frequency RF signal is applied to sampler drive signal input D1, a traveling electric field is piezoelectrically induced by a SAW on the surface of the GaAs substrate. Each potential "well" or lowpoint of the traveling electric field causes a sample of the signal on input signal input A1T1 to be pushed into FET channel F1. Accordingly, when an IF radar signal is applied to the signal input, each potential well causes an electron packet to be formed whose total electric charge is proportional to the instantaneous amplitude of the IF radar signal. These charge packets are carried through FET channel F1 at the fixed acoustic velocity (2864 meters/sec) of the transporting SAW. Output taps A1T2 overlapping FET channel F1 are used to sample the propagating charge packets nondestructively. A tap senses the electron density of a charge packet near it by sensing the electric field produced by the charge packet. In summary, a series of charge packets representing the amplitude of the IF signal over time are serially loaded into and moved through the FET channel at a fixed acoustic velocity.
Not only are ACT devices useful in building delay lines and filters, ACT devices can also be used in fashioning analog memories. A stationary electric field can be induced via interrupt field input port INT1 so that the traveling electric field of the SAW is overridden. If an IF input signal is sampled with the SAW generated propagating potential wells and if an overriding stationary electric field is then applied, the samples are held in a fixed position within the FET channel. It is possible to hold the packets in a fixed position for a relatively long period of time (up to milliseconds or seconds). Upon removal of the stationary electric field, the propagating SAW potential wells continue to move the charge packets through the FET channel as before. In this manner, the device forms a programmable delay line. Because the starting time and the duration of the interrupt stationary electric field can be locked to the radar system clock under digital control, the use of ACTs in clutter cancellers could provide precise stability and control.
If an ACT could be produced which could store an entire interpulse period, the ACT device could perform the function of the single pole clutter canceller depicted in FIG. 1. Unfortunately, the longest ACTs available have a FET channel length equivalent to 3 to 5 microseconds when typical interpulse periods range from 20 microseconds to 1000 microseconds.